A case of a patient who developed hyponatremia both during normal dose and during low dose carbamazepine therapy is described. The clinical characteristics of hypo-osmolar syndrome due to carbamazepine are similar to those of the syndrome of inappropriate antidiuretic hormone secretion. In literature, many cases of hyponatremia, related to assumption of normal dose carbamazepine, are reported; instead hyponatremia during low dose carbamazepine is rarely described. For this reason, a frequent monitoring of plasma sodium concentration, also in patients treated with low dose carbamazepine, is warranted.